1. Field of the Invention
This invention relates to a process for producing NH.sub.3 syngas in which a pressure swing adsorption unit and a fuel cell are employed.
2. Description of the Prior Art
The production of ammonia from natural gas is well known in the art. As described in U.S. Pat. No. 3,442,613, the disclosure of which is hereby specifically incorporated by reference, the following four chemical reactions are the principal reactions involved in a prior art ammonia process:
______________________________________ Reforming Reaction - CH.sub.4 + H.sub.2 O .fwdarw. CO + 3H.sub.2 (endothermic) Shift Reaction - CO + H.sub.2 O .fwdarw. CO.sub.2 + H.sub.2 (exothermic) Oxidation Reaction - H.sub.2 + 1/2O.sub.2 .fwdarw. H.sub.2 O (exothermic) Ammonia Synthesis - 3/2 H.sub.2 + 1/2N.sub.2 .fwdarw. NH.sub.3 (exothermic) ______________________________________
A conventional process for producing ammonia synthesis gas begins with the introduction of a feed stream, steam and air into a reforming stage which may involve one or two steps. Conventional primary reforming has been described, for example, in U.S. Pat. No. 3,132,010. In conventional secondary reforming, air is added to the effluent from the primary reformer to introduce nitrogen for the synthesis gas and oxygen which reacts with the combustible components to form oxides of carbon.
The reforming stage is followed by a purification stage, the first step of which is the slightly exothermic shift conversion reaction. This step conventionally involves high temperature catalytic conversion followed by low temperature catalytic conversion. The CO.sub.2 is then removed through a wash step. CO and CO.sub.2 which remain after shift conversion and washing are then converted into methane in a methanation reaction. Methane can be removed in a final purification step as outlined in U.S. Pat. No. 3,442,613 or allowed to concentrate in the synthesis loop and purged to fuel as in the prior art. A recycle stream from the downstream ammonia synthesis may be added between the reversed reforming reaction and the final purification step. The ammonia synthesis gas leaving the final purification step will have a trace of argon (less than 1%) and a hydrogen to nitrogen mole ratio of 3:1. However, this previous technology utilizes a number of steps, each of which requires careful maintenance and a sizeable initial capital investment, both of which are major variables in the cost of modern syngas production. Accordingly, there exists a need to reduce the cost to produce ammonia syngas by reducing the steps of the process while minimizing operational maintenance.